Those boxes have a lot of electronics inside them, basically one FPGA that is programmed to do all the conversion.
No, it's not even close. This would be like transplanting a dog's head on a human being. It takes a good bit of glue logic, and I suppose you could build all the glue yourself with TTL but you'd wind up with a big expensive box full of stuff.
What is wrong with the USB/RS-232C interface boxes that you don't like? Can you just put a serial card in your machine?
--scott
"C'est un Nagra. C'est suisse, et tres, tres precis."
[TELECOM Digest Editor's Note: I'd feel very sorry for the dog, if the animal lived through it all. PAT]